A problem of fault allocation in PON stems from the very nature of passive optical networks. Namely, at the PON physical layer, any information transmitted from one (central) Optical Line Termination (OLT) via an optical splitter, will be automatically shared between all fibers outgoing the splitter and terminating with Optical Network Terminations (ONTs). Similarly, information transmitted from any of the ONTs cannot be recognized by OLT as belonging to a specific ONT. This fact is due to the physical characteristics of the PON. Owing to that, if an optical fiber, terminating with a specific ONT, fails or degrades—OLT will be unable to recognize why the ONTs have stopped responding/transmitting. OTDR testing systems are based on analyzing power of returning optical signals, and on the fact that a fiber cut returns light of increased intensity. However, ONTs in PON networks are usually arranged in groups being fed from OLT through optical fibers via one or more optical splitters. Since upstream signals from ONTs are mixed with one another in the common optical fiber leading to OLT, and since the splitters always introduce high attenuation to the OTDR signal (normal attenuation number is 40 dB), the optical signals received from the ONTs are quite difficult for detecting and sorting properly. Any increase of power or duration of probe signals is also problematic due to limitations of the system, such as requirements to safety and resolution.
EP2086129A describes a method used for pinpointing (identifying, diagnosing) a faulty network terminal in a passive optical network (PON) comprising an optical line termination, a splitter upwardly connected the optical line termination (OLT) through a single fibre and network terminals (ONT) connected to the splitter. This method comprises sending from the optical line termination OLT a request to a probed network terminal for transmitting a testing signal, attempting from the requested network terminal to transmit the testing signal, analyzing the signal received at the optical line termination to detect the testing signal, determining whether the probed network terminal is a faulty terminal on the basis of the detection result. The method comprises quite a long interactive process. The active system at the subscriber is quite expensive since requires at least a TDM capability.
US2008304823A describes a technique notifying about determining a location of a fiber cut. For example, a method for determining a location of a fiber cut in a passive optical network (PON) including a plurality of optical network terminations (ONTs) is provided. According to the method, a plurality of base signatures are generated prior to the fiber cut. Each of the base signatures correspond to a known configuration of the plurality of ONTs. In response to the fiber cut, a current signature corresponding to a current configuration of the plurality of ONTs is generated. Whether the current signature matches one of the base signatures is determined. In response to finding a matching base signature, the location of the fiber cut is determined based on the known configuration of the plurality of ONTs corresponding to the matching base signature. The described method has a disadvantage that, for any specific fault in the network, a specific preliminarily generated signature should be posted in the data base of the system. Such a signature should be found and recognized in case that the specific fault occurs. Firstly, it requires a lot of effort from a network designed to predict and arrange signatures for any type of fault in the network and their processing (to build an incremental database, to collect information, to compare it with the database). Secondly, when the network is upgraded, the whole work of preparing signatures should be re-done from the very beginning, to take into account new portions of the network.